Accurate determination of Mg isotopes using MC-ICP-MS requires an adequ ate separation of elements from bulk samples. Cation exchange chromatography has been used to separate Mg from the matrix elements. However, the removal of Fe and Al from Mg solution is not effective, and Fe and Al do affect the accurate de termination of Mg isotope by MC-ICP-MS. Methods for different kinds of samples were investigated based on exchange chromatography using AG50W-X12 and coprecipitation of Fe and Al using diluted NH3·H2O, and Mg isotope composition of natural sample was determined after purification. The results show that ① It is enough to separate Mg from the bulk samples using the resin column one time when m(Fe)/m(Mg) ≤0.1, m(Al)/m(Mg) ≤0.2 and m(Na)/m(Mg) ≤1 in solution; ② It is enough to separate Mg from the bulk samples using the resin column two times when m(Fe)/m(Mg) ≤ 0.1, m(Al)/m(Mg)≤0.2 and m(Na)/m(Mg)＞1 in solution; ③ It is essential to obtain Mg solution using coprecipitation of Fe and Al by NH3·H2O and separation of Mg by cation exchange chromatography one time when m(Fe)/m(Mg) ＞0.1, m(Al)/m(Mg) ＞0.2 and m(Na)/m(Mg) ≤1 in solution; ④ It is essential to obtain Mg solution using coprecipitation of Fe and Al by NH3·H2O and separation of Mg by cation exchange chromatography two times when m(Fe)/m(Mg)＞0.1, m(Al)/m(Mg)＞0.2 and m(Na)/m(Mg)＞1 in solution; ⑤ The Mg isotopic compositions of the sea water around Qingdao City are δ25MgDSM3＝-0.43‰ and δ26MgDSM3＝-0.84‰; The Mg isotopic compositions of the sea water around HLD are δ25MgDSM3＝-0.44‰ and δ26MgDSM3＝-0.85‰.